Even if there were to suddenly be new doubt as to the constitutionality of certain long-standing US Government activities as John said the constitution would be amended in record time.

In terms of Congress spending money, I don’t think that has ever been in doubt. Using that money for back-door regulations, sure. But simply spending money? No, general welfare clause has made it clear that congress can spend money.

As to your second point, the Executive and Legislative branch have been going beyond the extremely strict and narrow interpretation of powers you seem to favor since the beginning.

In any case this is largely moot as well. None of the 3 branches seem inclined to agree with you, nor the several states, nor a majority of the people.

You can argue that 99% of what the Federal Government does is Unconstitutional but for the most part the courts haven’t agreed with you. Under the Constitution the courts pretty much have the final say on constitutionality barring a Constitutional Amendment.

“General” further implies that you can’t take taxes from one region and wholly direct it to a region without constitutional questions. The constitution itself bans taxing different states or regions at different levels. The 10th amendment covers neither such topic.

The General Welfare clause means that Congress can spend money on national interests. National interests not being explicitly defined, it seems like transportation can be one of them.

But I mean, the 16th amendment explicitly permits congress to have taxing authority. Congress explicitly makes laws and thus determines the spending of that money. The Federal Government can indeed offer states, regions, cities, companies, and individuals money but cannot compel their acceptance. For example, it would likely be unconstitutional to force Seattle to build a transit system with our own money.

The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.

Intentional vague like all great government guiding principles I fail to see how this precludes the states, through representation in Congress, from collecting funds and distributing in any manor they so see fit. If States refuse those funds (and the attached strings) then so be it. We’ve seen examples of this in the current stimulus.

But it doesn’t, of course, according to the Supreme Court. So if we’re going to spend money it should be done on merits and not selectively choosing to enforce odd interpretations of the 10th amendment. Obviously states have a right not to build transit, the federal government doesn’t force them to, it just offers money in case they do.

Funding transit could be positioned as in the interests of national defense by saying it helps decrease our dependency on foreign oil. I’m not sure that’s the case but it’s certainly as arguable as continued funding of the interstate highway system being essential for national defense.

Local roads and local transit should be paid for by state and local.

Does that mean the construction of I-5 through Seattle should be fully paid with local money instead of the 90% federal funding they got back in the 60s? It is called an interstate system yet nearly twice the vehicle-miles travelled on interstates occur within urban areas for commuters than for rural and interstate purposes . Also, federal money for highways spent in urban areas places locally funded transit at a disadvantage. It was only in 1964 when mass transit was given federal support and in 1973 when highway funds could be used for mass transit capital projects with up to 80% federal matching.